The present invention relates to a method of depositing silicon onto a carbon material such as carbon nanofibers or composites formed from carbon nanofibers to form an alloy which can undergo lithiation/delithiation and which may be used as an anode in lithium ion batteries.
The use of lithium ion batteries as rechargeable power sources represent a promising technology for use in the development of consumer electronics and electric-based vehicles as they can replace traditional aqueous batteries such as lead-acid, nickel metal hydride, nickel-cadmium, and nickel hydride batteries.
Current lithium ion batteries typically use lithium cobalt oxide as the cathode and carbon or graphite as the anode. Efforts have been made to increase the energy density and power capability of the anode in lithium-ion batteries to provide improved operating features for electric and/or hybrid-type vehicles, cordless power tools, and electronics. For example, recent research indicates that anodes formed from nanocarbon materials can provide increases in both energy storage and power capability. Some single-multi-walled carbon nanotubes have shown reversible intercalation capacities in excess of LiC6.
Another area of interest in recent years has been the investigation of metals or alloys that will form alloys with lithium, as such materials are known to store as much as eleven times the energy of current negative electrodes made of carbon alone. Silicon, which has a theoretical capacity of up to 4200 mAh/g, is one such material. Carbon-silicon alloys have previously been formed through various milling processes and through solution deposition of siloxanes onto graphite.
However, the use of such carbon-silicon alloys has been limited for use in lithium batteries as they undergo a significant volume change as they incorporate and release lithium during charge and discharge. As silicon undergoes an approximate 300% volume expansion when fully charged, alloys containing silicon can fragment and lose electrical contact with the anode as the result of these volume changes. This phenomenon is particularly destructive when the active materials are in the form of particulates, frequently resulting in a rapid loss of capacity upon cycling.
Furthermore, the development of batteries designed for high charge/discharge rates show evidence of heat retention in the battery cell, which can ultimately degrade the performance of the battery cell. High thermal conductivity composites have been fabricated which facilitate heat transfer through the composite. See, for example, U.S. Pat. No. 5,837,081, which teaches the use of vapor grown carbon fibers to fabricate high thermal conductivity composites. Use of high thermal conductivity materials in the fabrication of the anode would serve to eliminate heat retention or heat build-up within the battery cell as it is subjected to high charge and discharge rates.
More recently, surface modification of carbon fibers has been achieved by coating with materials such as silicon to provide a high thermal conductivity network and provide the ability to survive repeated thermal cycling. See, for example, U.S. Pat. No. 6,988,304, which teaches modification of vapor grown carbon fibers for the purpose of forming a composite structure for containing a phase change material for use in aircraft brakes. It would be desirable to use a surface modification process on carbon substrates or composite preforms for use in lithium ion batteries.
Accordingly, there is still a need in the art for a method of modifying carbon materials which can be used to make an improved anode for use in a lithium ion battery.